Applied an organic protective coating on the inside of a carbon steel (a material conforming to API 5CT provision) oil casing (abbreviation for “oil tubing and production casing”) can effectively extend the life of the carbon-steel oil casing, such coating protection technology has been widely used in oilfields around the world. An oil casing having an inner coating is utilized more than one million meters per year. There exist a plurality of logging techniques used to solve problems, such as corrosion, damage and leakage within a body of an oil casing. Among which, the relatively well-developed logging methods widely applied in the industry mainly include types of downhole electromagnetic thickness logging, multi-armed caliper logging and temperature logging, optical television imaging, ultrasonic imaging, and the like.
Raw materials of the inner coating of the oil casing mainly include organic coatings, such as a modified epoxy phenolic, an epoxy phenolic, or the like, which are often cured on an inner surface under an ambient temperature or a high temperature to form a dense coating having a thinner thickness generally of from 0.15 mm to 0.50 mm (from 150 μm to 500 μm). Prior to the downwelling of an oil casing, it is possible to determine whether there exist quality issues of the inner coating by way of visual inspection, endoscopy, leakage point, insulation detectors, and the like, but after the downwelling of the oil casing, the oil casing also has complicated medium such as completion fluids, oil gas water and the like therein under the ground generally at hundreds to thousands of meters in depth. As the well depth increases, the well has a certain temperature and pressure therein. In this case, it is quite difficult to determine and detect whether the oil casing has an inner coating, and the length of and quality of the inner coating.
A conventional downhole electromagnetic thickness logging technology cannot produce any electromagnetic signal feedback to organic coatings.
A multi-armed caliper logging system can detect changes in the inner diameter of an oil casing. At present, the most advanced Sondex multi-armed caliper logging system achieves a radial inspection of an oil casing with the highest accuracy of only ±0.762 mm (±762 μm), which is greater than the thickness of the inner coating, thus it fails to detect the damage condition of the inner coating of the downhole oil casing, and the scales, corrosion products or wax on the inner surface of the oil casing can further affect detection accuracy.
A well temperature logging mainly determines changes in flow field temperature of the fluid produced in reservoirs of oil wells or the fluid from an upper strata invaded into the wells, and thus it is impossible to detect the presence of the inner coating of the oil casing.
It is very difficult for an optical television imaging logging system to accurately determine whether there exists an inner coating of an oil casing under the influence of the cleanliness of the surface of the oil casing, an intensity of illumination and a viewing angle of an instrument in limited space.
Although an ultrasonic imaging logging system can be used for quality evaluation of a casing and cementing, and the thickness of the wall of a carbon steel casing in the relevant publications has a radial resolution of up to 0.05 mm (50 μm), the logging and interpretation procedures are complicated with high cost, and the system has not yet been utilized in detection of an oil casing having an inner coating of oil wells.
A conventional resistivity logging method is widely used in open hole completion, of which a primary objective is to carry out a sequence determination and a sequence division of downhole rock stratum having different physical properties. The existing method cannot implement an effective detection in a carbon steel oil casing, the oil wells using a casing having an inner coating often use a casing without an inner coating in a specific section below wellhead, and use a casing having an inner coating in local sections above the bottom, thus it is quite difficult to implement an accurate detection and determination; the principle of lateral focusing method for measuring resistivity is feasible, but it presents a complicated circuit with high mechanical cost. The result of adopting a spot radiation source is a lump-type inspection, and the damage rate cannot be determined.
For detection of qualities of the pipelines having an inner coating or the inner coating of the oil casing on the ground, there exist well-developed and effective methods, such as a low pressure leak detection method, an online electric spark leak detection method, an endoscope method, and the like. For detection and analysis of the damage rate of an inner coating of a downhole oil casing, such research results or patent release have not yet been published.